One species that describes the class secernentea is the hookworm. Hookworms are long and skinny worms that are unsegmented. they come in many different sizes, ranging from as small as microscopic to as long as one meter. They have three germ layers that consist of the endoderm (innermost layer), exoderm (outermost layer), and mesoderm (middle layer). Something that is unique about the hookworm is the cavity between the mesoderm and endoderm tissues, which is called a pseudocoelom, or "false coelom." the structure of a roundworm can be described as a tube-within-a-tube. The inside tube is the digested tube and the outside tube is the body wall, that keeps everything together.